It turns out this was a known bug, fixed in Nevada build 96: 6714234 luactivate has problems with ZFS root in non-C locales
The workaround is to force luactivate to use the "C" locale. I hardcoded settings for the "C" locale in /etc/default/lu in both the old and the new boot environment and after that it was possible to activate the new boot environment. Thanks to all who responded. /Magnus
